1. Field of the Invention
The present invention relates to an apparatus for automatically tightening various kinds of male screws into the corresponding mounting holes formed in a workpiece.
2. Related Art
For automation of screw tightening processes in recent years, it is required to provide technology for tightening at high speed various kinds of male screws into female screw holes formed in a workpiece and for decreasing the defect rate in the tightening of the male screws. In a conventional screw tightening apparatus, for example, an electric screwdriver is arranged to be moved by a pneumatic cylinder in a vertical direction, and an electric motor of the screwdriver is arranged to be supplied with electric current from an electric power source in response to an electric signal to start screw tightening. In operation, the screwdriver is moved downward by operation of the pneumatic cylinder, and the bit shaft of the screwdriver is driven by the electric motor to tighten a male screw into the corresponding female screw hole with a predetermined torque.
In the conventional screw tightening apparatus, however, vertical movement of the screwdriver may not be controlled in an appropriate manner. For this reason, it is a distinct possibility that the opening end of the female screw hole is damaged by abutment with the male screw to cause abnormal engagement therewith. Since the bit shaft of the screwdriver is driven at a constant speed, the working time for screw tightening is inevitably prolonged if the rotation speed of the screwdriver is decreased to reduce the defect rate of screw tightening. If the rotation speed of the screw driver is increased to shorten the working time for screw tightening, the occurrence of abnormal engagement of the male screw with the female screw hole will increase, and the accuracy in torque applied to the male screw may not be stabilized due to inertia of the screwdriver caused by increases of rotation speed of the electric motor.
To overcome the foregoing problems, it has been proposed in Japanese Patent Laid-open Publication No. 64-11740 to change the rotation speed of the screwdriver under control of a timer for conducting initial tightening at a high speed and final tightening at a low speed. In such control of the screwdriver, however, the tightening speed of the male screw may not be controlled in accordance with the tightening condition of the male screw. It is, therefore, difficult to detect the start of engagement with the female screw hole. If the bit shaft of the screwdriver started to rotate at a high speed before the male screw is brought into engagement with the female screw hole, normal engagement of the male screw would not be expected, resulting in a defect of screw tightening. In the case that the rotation speed of the screwdriver is controlled in accordance with engagement of the male screw with the female screw hole without proper control of thrust force acting on the bit shaft, there will occur a defect in the screw tightening.
In Japanese Patent Laid-open Publication No. 3-202236, it has been proposed to detect initial engagement of the male screw with the female screw hole on a basis of an input current applied to the electric motor of the screwdriver for controlling the rotation speed of the bit shaft. The input current of the electric motor is, however, increased at a time when the male screw is brought into engagement with the female screw hole of the workpiece. This means that the timing for increasing the input current does not correspond with initial engagement of the male screw with the female screw hole. It is, therefore, apparent that if the bit shaft of the screwdriver was driven at a high speed in spite of poor engagement of the male screw with the female screw hole, there will occur defective screw tightening.
For the purpose of overcoming the problems mentioned above, the inventors have found that the following facts are required for shortening the screw tightening time and for decreasing the defect rate of screw tightening.
1) Finishing the screw tightening in a short time.
2) Avoiding the occurrence of defective screw tightening, which causes chafing of the male screw in engagement with the female screw hole, crushes the screw head, or allows free rotation of the male screw by stripping the threads or the like.
3) Detecting the occurrence of defective screw tightening.
4) Correcting a teaching point to avoid defective screw tightening.
In case the rotation speed of the bit shaft is increased to shorten the screw tightening time, there will occur defects of screw tightening such as chafing of the male screw in initial engagement with the female screw hole, decrease of accuracy in torque control, damage of the male screw head or the like. To avoid such problems, the inventors have specifically analyzed the above phenomena and found that during screw tightening, if time can be shortened without any defective screw tightening, the rotation speed of the bit shaft is decreased at initial engagement of the male screw and prior to increase of tightening torque and is increased at the other occasion.
Furthermore, defective screw tightening is caused by various phenomena such as poor engagement of the bit shaft with the male screw, crushing of the male screw head or the female screw hole or the like. These phenomena will occur if an advance force applied to the male screw is inappropriate in tightening into the female screw hole. As a result of analysis of the phenomena, it has been found that the thrust force acting on the male screw head must be controlled to be 20-30% of the tightening torque without causing disengagement of the bit shaft from the male screw head. If the male screw is applied with an excessive thrust force before it is brought into engagement with the female screw hole, the female screw hole will be damaged or stuck in engagement with the male screw. Accordingly, it is desirable that the thrust force of the bit shaft is controlled in accordance with the following steps of:
1) advancing the male screw to bring into engagement with the female screw hole,
2) seating the male screw on the workpiece after engagement with the female screw hole, and
3) increasing the tightening torque of the screwdriver after the male screw was seated on the workpiece.
For such control of the thrust force, it is necessary to detect a tightening condition of the male screw. Since the tightening torque changes only in slight amount before and after engagement with the female screw hole, it is impossible to detect the start of engagement of the male screw with the female screw hole by the torque or rotational position of the bit shaft. It is, however, apparent that the distal end of the bit shaft is placed in the same position if the male screw freely rotates when advanced by engagement with the bit shaft. When the male screw is brought into engagement with the female screw hole, the bit shaft is advanced by the pitch of the male screw thread. Accordingly, the start of engagement with the female screw hole can be detected by movement of the bit shaft. Since the torque of the bit shaft increases when the male screw is seated on the workpiece, the seating of the male screw can be detected by change of the torque of the bit shaft. However, the accuracy of detection of the seating may not be ensured under high speed rotation of the bit shaft. It is, therefore, necessary to decrease the rotation speed of the bit shaft prior to seating of the male screw. For control of the rotation speed of the bit shaft, the seating time of the male screw can be estimated on a basis of the initial engagement time of the male screw, the rotation speed of the bit shaft and the pitch of the male screw thread.